It is not uncommon for undetected bleeding to occur during surgical procedures because of an unintentionally severed vein or artery. The ensuing loss of blood can result in serious deterioration in the patient's condition, or even in death, if it is not promptly stemmed.
Blood pressure and pulse rate are monitored by the anethesiologist during a surgical procedure, and these parameters provide valuable information on the patient's condition. However, because systemic vascular resistance can increase dramatically during episodes of blood loss such as those just described, it may be as long as four or five minutes after serious bleeding develops or a blood vessel is severed before an appreciable decrease in pulse rate or blood pressure occurs. Currently, serious undetected losses of blood can occur in periods of this magnitude because as much as 35 percent of a patient's blood may be lost before there is a noticeable decrease in blood pressure. By then, however, the patient may be going into shock or suffering other complications attributable to the loss of blood. Although blood pressure and pulse may remain relatively constant for this extended period of time, cardiac output begins to decrease coincidentally with the loss of blood. Hence, by monitoring this parameter, loss of blood can be detected much earlier than would otherwise be the case. This permits the surgical team to take prompt remedial action, hopefully forestalling the deterioration in the patient's condition that might have occurred had the loss of blood gone unchecked.
Variations in cardiac output can also be utilized to detect other unwanted changes in the patient's condition such as the onset of ischemia of the heart muscle or an anesthetic reaction, again permitting remedial action to be taken before there is any significant deterioration in the patient's condition.
Thermal dilution is one technique which has heretofore been employed to measure cardiac output. In that technique, a thermal dilution catheter carrying a thermistor on its tip is inserted through an incision into the jugular vein and threaded through that vessel and the right side of the patient's heart into the pulmonary artery. A saline solution is then injected through the catheter into the patient's bloodstream, typically at a temperature of 0.degree. C. This solution mixes with the blood flowing through the pulmonary artery, momentarily reducing the temperature detected by the catheter tip thermistor. Standard thermodynamic equations allow the cardiac output to be determined from this drop in temperature and the volume of saline solution which produced the temperature drop.
The thermal dilution technique of measuring cardiac output has the disadvantage that it is highly invasive and therefore potentially capable of damaging the anatomical structures through which the catheter is threaded. In fact, in a small percentage of cases (one to two percent), serious complications result from employment of the thermal dilution technique.
Also, the mere presence of the catheter in the pulmonary artery may result in localized clotting of the blood flowing through that vessel. This can obstruct the orifice through which the saline solution is discharged or produce an insulating layer around the thermistor. In both cases, the results will be highly inaccurate.
Finally, the thermal dilution technique is time consuming as it may take as long as 30 minutes to place the catheter; and only a limited number of measurements per hour of cardiac output can be made. Changes in a patient's condition requiring prompt remedial action may therefore not be detectable by the thermal dilution technique.
Because of the disadvantages discussed above, the thermal dilution technique for measuring cardiac output is generally employed only if the patient is undergoing cardiac surgery or is sufficiently ill that surgery poses a risk of cardiac failure.
The Fick method is another technique for measuring cardiac output that has heretofore been employed to some extent. In it, blood samples are taken at two different points in the circulatory system, one just downstream of the patient's aorta and the other in the pulmonary artery. The concentrations of oxygen in these arteries are compared, and the resulting value is combined with the amount of carbon dioxide being expelled by the patient to provide a measurement of cardiac output.
The Fick technique has the disadvantage that the measurements are complex and can easily require a day of analysis before cardiac output can be ascertained. This makes the Fick technique useless in the operating theatre where up-to-the-minute information is required to keep the patient in a stable condition.
Of the techniques for measuring cardiac output discussed above, thermal dilution is the most widely employed.
The drawbacks and disadvantages of the above-discusssed techniques for measuring cardiac output are eliminated in the method of measuring cardiac output described in above-cited Patent No. 4,509,526.
In the method of measuring a patient's cardiac output disclosed in the '526 patent, the diameter of the patient's ascending aorta is determined by a pulsed-echo transducer placed on his or her chest, and the systolic velocity of the blood flowing through that artery is determined by insonification of the aorta with an ultrasonic suprasternal notch probe. This second, also external probe makes available Doppler or frequency-shifted electromagnetic signals which are analyzed and converted from the time domain into discrete frequency components by digital fast Fourier transform. The Doppler shifted frequency components of the return signal are converted to velocities, and the latter are employed to calculate a systolic velocity integral.
Multiplying the systolic velocity integral by the cross-sectional aortic area yields beat-by-beat cardiac stroke volumes of the patient; summing the stroke volumes over a predetermined number of consecutive beats and then dividing by the time spanning the predetermined number of beats (in other words, multiplying by the heart rate) yields the patient's cardiac output.
The patented cardiac monitoring apparatus facilitates direct operator interaction with the apparatus over the course of the measurement protocol via a touch sensitive visual display which, inter alia: instructs the operator at each step of the sequence and responds to the election of operator options with failsafe features that guard against the entry of invalid data and otherwise minimize operator error. The operator may interact without extensive training, and the system provides the benefits of microprocessor control including fast data processing without elaborate hardware or software.
Within operational limits, the patented cardiac monitoring system will insist upon the entry of required data, will limit the entry of certain data to values within statistically anticipated ranges, and will assist the operator in optimizing the measurement of variable parameters.
The method for measuring cardiac output which is practical with the apparatus disclosed in the '526 patent is significantly superior to any previously available techniques for providing this vital information. The patented technique for determining cardiac output is noninvasive and therefore does not subject the patient to the risk of infection or anatomical damage or require surgery as is the case in those cardiac output measuring techniques employing a catheter. And the patented method permits cardiac output to be monitored on a continuous, up-to-the present-moment basis. Nevertheless, the patented technique does have its disadvantages. The equipment is expensive and heavy, and extensive operator training is required.
Pending application No. 763,992 discloses a technique for measuring cardiac output and related parameters which eliminates the drawbacks of the approach disclosed in the '526 patent. The equipment utilized in carrying out the method is simpler; less expensive; and easier to operate, significantly reducing the amount of operator training that is required to use it.
In the novel method and apparatus for monitoring cardiac output to which the '992 application is devoted, an ultrasonic esophageal probe is substituted for the suprasternal notch probe used to monitor systolic velocity in the system disclosed in the '526 patent. This probe monitors the blood flowing through the patient's descending aorta rather than his ascending aorta. This velocity is scaled to the velocity of the blood flowing in the patient's ascending aorta, and the result is combined with a number representing the area of the patient's ascending aorta to produce a cardiac output value.
This substitution of an ultrasonic esophageal probe for the suprasternal notch probe employed in the patented equipment is important when the system is used during surgery. The preferred type of esophageal probe does not interfere with the operating field as does a suprasternal notch probe of the type disclosed in the '526 patent. In addition, unless esophageal surgery is involved, the probe is out of the sterile field, which is an obvious advantage. Furthermore, this probe replaces the esophageal stethoscope which would be employed in any event so that, in effect, another measurement of the patient's condition can be monitored without further invasion of the patient's body.
The blood flowing through a patient's descending aorta is only about 70 percent of that flowing through his ascending aorta, the remainder having been distributed to the patient's subclavian and carotid arteries before the descending aorta is reached. Consequently, in the cardiac output measuring apparatus disclosed in the '992 application, provision is made for scaling the systolic velocity measured with the esophageal probe by an appropriate conversion factor to the velocity which would have been obtained if the flow in the patient's ascending aorta were instead monitored.
The proportioning of the blood pumped by a patient's heart between the descending aorta and those other blood vessels discussed above will vary from patient-to-patient. Consequently, the suprasternal notch probe technique of measuring systolic velocity disclosed in the '526 patent is preferably used to determine an accurate conversion factor for each patient.
The technique for providing the aortic area value that is disclosed in the '992 application is also completely different from the patented technique. In the latter, aortic diameter is measured by insonification of the patient's ascending aorta and converted to aortic area. The approach described in the '992 application instead employs a predictively determined value of aortic diameter for determining cardiac output. This has the advantage that it makes the cardiac monitoring equipment much simpler to use, lighter, and less expensive than that disclosed in the '526 patent.
After the apparatus disclosed in the '992 application was put into widespread clinical use, it was found that the aiming of the esophageal probe by the provided for techniques sometimes led to less than optimal results. This is because the aiming technique in question made use of a signal related to the peak velocity of the blood flowing through the patient's descending aorta, and it was proved that the peak velocity may occur in a region well removed from the centerline of the descending aorta while a transducer aimed at the aortic centerline produced a signal which, when converted to cardiac output, most accurately reflected the patient's actual cardiac output.